Halliday & Matthiessen (2014: 490):
There is a gradual loss of information, in the way a process is construed in the grammar, as one moves from the finite free clause to the prepositional phrase; for example ‘soon you will reach the monument; then continue straight ahead’:
(1) shows transitivity, with Process and Medium; bound mood, with Subject, and primary tense (system I). (2) shows transitivity, with Process and Medium; free mood, with Subject, and reduced primary tense (system II). (3) shows transitivity with Process but no Medium; no mood, and no explicit Subject; no primary tense (system III). (4) shows no transitivity (minor process only), no mood, and no tense.